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An Unusual (and Promising) Brown Dwarf Detection

The naming of names is quite a project when it comes to new astronomical objects, and given the sheer numbers — 300 million habitable planets around G- and K-class stars, for example — we might do better to stick with simple identifiers. On the other hand, it’s a bit charming that a new brown dwarf known by its identifier as BDR J1750+3809 has been dubbed ‘Elegast’ by the discovery team. This is the first substellar object found through radio observations.

The name is both appropriate and specific to the discovery space. Elegast appears in a poem in Middle Dutch (12th or 13th Century) called ‘Karel ende Elegast,’ with the character Elegast being a vassal of Charlemagne who seems to be king of the elves (Wikipedia to the rescue, vindicating once again my decision to send them a monthly donation).

The Dutch connection is that the radio work comes out of LOFAR (Low-Frequency Array), which is currently the largest radio telescope operating at the lowest frequencies that we can observe from Earth. Based in the Netherlands, LOFAR is an international effort with field stations in eight countries.

Elegast breaks from the norm in not being discovered from the kind of infrared sky surveys through which most other brown dwarfs have been found. You might recall our discussions of the WISE mission and the possibility, not realized, of finding a brown dwarf closer than the Alpha Centauri stars. But it’s also true that infrared surveys can’t detect objects that are sufficiently cold and faint, whereas we now have radio wavelengths to fill the gap.

Is there still some faint possibility of a closer brown dwarf? T-class dwarfs are defined as being less than 1500 Kelvin, but WISE identified Y-dwarfs with temperatures as cool as 25 °C (298 K), which gets us down to where my thermostat is set as I write this. WISE 1828+2650, I subsequently learned, is so cold that it emits no visible light at all and is more or less indistinguishable from a free-floating planet. Get any colder and infrared surveys won’t find such worlds, which is why the news about Elegast is so encouraging.

Given that the population of so-called ‘rogue’ planets may be vast, it’s wonderful to see a new method becoming available with which to study nearby examples. Brown dwarfs may not be able to create the kind of fusion reactions found in stars like the Sun, but they emit at radio wavelengths just as gas giant planets do. Jupiter’s emissions are well studied, the result of powerful magnetic fields around the planet accelerating charged particles to produce radio waves as well as aurorae. Brown dwarfs are faint at these wavelengths but detectable.

Image: Artist’s impression of Elegast. The blue loops depict the magnetic field lines. Charged particles moving along these lines emit radio waves that LOFAR detected. Some particles eventually reach the poles and generate aurorae similar to the northern lights on Earth. Credit: ASTRON / Danielle Futselaar.

The LOFAR observations had to contend with background galaxies when investigating potential brown dwarfs. The key turned out to be identifying signals that were circularly polarized, which distinguishes planets and brown dwarfs from the background ‘noise.’ What this kind of polarization means is that the electromagnetic field of the radio waves rotates in a characteristic circular pattern. Says LOFAR project scientist Tim Shimwell: “We could not have picked out Elegast in our standard radio images from among the crowd of millions of galaxies, but Elegast immediately stood out when we made circularly polarised images.”

In the case of Elegast, the Gemini-North Telescope had produced archival imagery used to confirm the discovery, and the NASA Infrared Telescope Facility (IRTF) deployed an infrared spectrometer known as SpeX, recently upgraded, to obtain a spectrum of Elegast. The signature of methane, common in brown dwarfs and gas giant planets, was revealed.

Beyond its possibilities at identifying the faintest brown dwarfs and free-floating planets, radio astronomy may also be used to study exoplanet magnetic fields, testing theories on magnetic field strength on objects that have enough similarities to gas giants to make the investigation worthwhile. The team is now making follow-up observations on Elegast to measure its magnetic field even as they use LOFAR to look for more such objects. Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON) is lead author of the study:

“Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life. Because magnetic phenomena of cold brown dwarfs like Elegast are so similar to what is seen on solar system planets, we expect our work to provide a vital datapoint to test theoretical models that predict the magnetic fields of extrasolar bodies.”

The paper is Vedantham et al., “Direct radio discovery of a cold brown dwarf,” Astrophysical Journal Letters Vol. 903, No. 2 (9 November 2020). Abstract / Preprint.

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{ 6 comments… add one }
  • Christian G November 12, 2020, 17:23

    Given you mention the search for cool objects closer than Alpha Centauri, it might be relevant to mention the distance to Elegast: 180-240 ly.

  • Bruce D Mayfield November 12, 2020, 23:56

    Is it at all possible that LOFAR could detect our elusive planet nine? Can a distant ice giant or super earth orbiting the Sun still have a magnetic field after 4.5 billion years?

  • Michael Fidler November 13, 2020, 7:15

    The LOFAR found this signal on 144 MHz which is in the 2-meter amateur radio band. Could observation survey’s be done with other radio telescope or a specialized designed telescope for circularly polarized work. There is a lot of equipment available for this radio band so how hard would it be to set up an dedicated array to look for these signals?

  • Jeffrey Stopple November 13, 2020, 12:37

    You do know who Charlemagne was in real life, right?

    • Paul Gilster November 13, 2020, 14:50

      I’m chuckling because in real life, I did my grad work on literature of the medieval era, focusing on the early Anglo-Saxon poets. Charlemagne loomed large in the popular imagination throughout Europe as the Middle Ages progressed, so I encountered him quite a bit, especially when diverging into the medieval romances as a break from Anglo-Saxon. But I had never encountered Elegast before.

      Charlemagne is indeed a historical figure, and a hugely significant one. But many historical figures were melded into tales of fantasy and romance in the Middle Ages. It became a rich tradition that evidently turned up in the Elegast story as another example. I was glad to learn about Elegast.

      • Andrei November 14, 2020, 22:04

        Never know what to learn here, this time that Charlemagne did become such a myth he was an elf king. :)
        LOFAR is one very interesting instrument, congratulations to the team for this discovery. The chance of finding a red dwarf or bright brown dwarf is fairly slim, but one Y class would very well have avoided detection. There more possibility and that we might even have observed one thought be to be extremely faint and distant. distant. But it is one that moved in the same direction as the sun and so did not stand out. Someone will say that I am wrong.
        But the distance to very often observed Betelgeuse turned out to wrong and that star smaller than thought for a very long time.

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